1. Field of the Invention
The present invention is presented to a shock wave source for use in extracorporeal lithotripsy and in particular to such a shock wave source having a plurality of electro-acoustic transducers arranged along a concave surface, which can be driven in a pulsed fashion for generating shock waves in a propagation medium disposed between the transducers and a patient.
2. Description of the Prior Art
In extracorporeal lithotripsy, a shock wave source is pressed against the body of a patient, in which a calculus is disposed, with a flexible membrane of the shock wave source functioning as a coupling agent. A suitable locating system is used to insure that the calculus to be disintegrated is located at the focus of the shock wave source. The calculus disintegrates into fragments by the action of the shock waves emanating from the shock wave source, and these fragments can be eliminated in a natural manner.
A shock wave source is described in German OS 33 19 871, corresponding to British Specification 21 40 693, wherein a plurality of electro-acoustic transducers are disposed along a concave surface. Each of the transducers can be individually driven in a pulsed manner, to generate shock waves in a propagation medium disposed between the transducers and the patient. Each transducer has an acoustic axis, and the shock wave source as a whole also has an acoustic axis. The acoustic axes of the transducers intersect at a focus which lies on the acoustic axis of the shock wave source. The transducers are arranged on a surface which is a portion of a sphere, so that the focus of the shock wave source corresponds to the center of curvature of this surface. Consequently, the transducers in this shock wave source are positioned a relatively large distance from the body surface of the patient, if the calculus to be disintegrated is disposed close to the body surface. Because this known shock wave source has a constant aperture angle, which is defined by the radius of curvature of the surface on which the transducers are arranged and by the diameter thereof, the shock waves must be coupled to the body of the patient via an extremely small region of the body surface. This results in an undesirably high power density at this location of the body surface, which may be injurious under certain circumstances.
If locating of the calculi is undertaken with an ultrasound locating system disposed in the center of the spherical surface on which the transducers are disposed, further disadvantages result. If the ultrasound locating means is disposed so that it can be applied to the body surface of the patient with only the interposition of the coupling membrane, as is most desirable for obtaining accurate ultrasound images, the ultrasound probe occupies a considerable portion of the region of the body surface available for coupling of the shock waves during treatment. This means that the power density at the remaining portion of the body surface available for treatment must be further increased in order to assure success of the treatment. If the ultrasound probe is moved away from the body surface so that an adequately large region of the body surface is available for shock wave treatment, reflections of the ultrasound waves emitted by the ultrasound probe will arise at the coupling membrane, thereby resulting in image artifacts in the ultrasound image, making locating of the calculus to be disintegrated more difficult, or impossible.
Another shock wave source is disclosed in German OS 31 19 295, corresponding to U.S. Pat. No. 4,526,168, wherein the transducers are driven with a chronologically offset signal so that the shock waves emitted from the individual transducers simultaneously arrive at the focus of the shock wave source. This known shock wave source has a control unit which acts on the drive system for the transducers, so that the chronological offset is variable and the focus of the shock wave source can thus be displaced along the acoustic axis of the shock wave source. This known shock wave source has a focal distance which is electronically variable, i.e., with electronic focusing. This permits the shock wave source to the operated with a small focal distance, and thus with a large aperture angle, for treatment of calculi lying close to the body surface of the patient, so that the power density at the body surface can be held within tolerable limits. If an ultrasound locating system is used, this can be applied to the body surface of the patient, with only the coupling membrane being interposed therebetween, at the same time as treatment, without the region of the body surface available for coupling of the shock waves being diminished due to the positioning of the ultrasound probe. In this known shock wave source, however, the extent of each transducer transversely relative to the direction of its acoustic axis cannot exceed 1/8th of the wave length of the shock wave emitted by the transducer. If this condition is not observed, excessive transit time differences will result at the focus between those components of the shock wave which are respectively emitted from the edge of the transducer and from the center of the transducer. Effective focusing would be impossible under those conditions. Consequently, this known shock wave source must have an extremely high number of relatively small transducers in order to generate shock waves having adequate energy and an adequate degree of focusing. This results in a complicated structure for the shock wave source itself, and also requires a complicated drive system and a complicated control system for the transducers. Additionally, sufficient electrical strength cannot be guaranteed, with an economic material outlay, when such extremely small transducers are used.